An acetyl moiety was discovered as the amino-terminal blocking group of viral coat protein in 1958 (Narita, K., Biochim Biophys. Acta 287:184-191 (1958) and of hormonal peptide in 1959 (Harris J. I., Biochem. J. 71:451-459 (1959). Since then, a large number of proteins in various organisms have been shown to possess acetylated amino-terminal residues. For example, mouse L-cells and Ehrlich ascites cells have about 80% of their intracellular soluble proteins No-acetylated (Brown, J. I. and Roberts, W. K., J. Biol. Chem. 251:1009 (1976) and Brown, J. I. J. Biol. Chem. 254:1447 (1979)). In lower eukaryotic organisms, about 50% of the soluble proteins are acetylated (Brown, J. L., Int'l Congr. Biochem. Abstr. (Internation Union of Biochemistry, Canada) Vol. 11:90 (1979)). These data demonstrate that N.sup..alpha. -acetyl is a very important blocking group. It has been suggested that the biological function of this blocking group may be to protect against premature protein catabolism (Jornvall, H., J. Theor. Biol 55:1-12 (1975)) and protein proteolytic degradation (Rubenstein, P. and Deuchler, J., J. Biol. Chem. 254:11142 (1979)). However, in mouse L-cells such N.sup..alpha. -acetylation does not apparently have this biological function (Brown, J. L., J. Biol. Chem. 254:1447 (1979)).
Although a clear general function for N-acetylation has not been assessed with certainty, some specific effects for a small number of proteins have been observed. Nonacetylated NADP-specific glutamate dehydrogenase in a mutant of Neurospora crassa is heat-unstable, in contrast to the acetylated form (Siddig et al., J. Mol. Biol. 137:125 (1980)). A mutant of Escherichia coli, in which ribosomal protein S5 is not acetylated, exhibits thermosensitivity (Cumberlidge, A. G. and Isono, K., J. Mol. Biol. 131:169 (1979)). N.sup..alpha. -acetylation of two of the products from the precursor protein proopiomelanocortin has a profound regulatory effect on the biological activity of these polypeptides; the opioid activity of .beta.-endorphin is completely suppressed, while the melanotropic effect of .alpha.-MSH is increased if N.sup..alpha. -acetylated (Smyth et al., Nature 279:252 (1970); Smyth, D. G. and Zakarian, S., Nature 288:613 (1980); and Ramachandran, J. and Li, C. H., Adv. Enzymol. 29:391 (1967)). Both acetylated and nonacetylated cytoplasmic actin from cultured Drosophila cells participate in the assembly of microfilaments, the latter, however, with less efficiency (Berger et al., Biochem. Genet. 19:321 (1981)). More recently, the rate of protein turnover mediated by the ubiquitin-dependent degradation system was shown to depend on the presence of a free .alpha.-NH.sup.2 group at the N-terminus of a protein (Hershko et al., Proc. Nat'l Acad. Sci. U.S.A. 81:9021-9025 (1984) and Bachmair et al., Science 234:179-186 (1986)), suggesting that N.sup..alpha. -acetylation may have a role in impeding protein turnover.
Given the importance of N-acetylation for the function and the ability of these N-acetylated proteins to modulate cellular metabolism, it is of interest to examine the subcellular location, substrate specificity, and regulation of protein acetyltransferase. In order to cast light on the biological implications of the acetylation and to elucidate the enzymatic mechanism of the reaction, as a first step, an enzyme must be isolated that is able to catalyze the amino-terminal acetylation in order to investigate its substrate specificity. The existence of such an acetyltransferase has been demonstrated and studied in E. coli for ribosomal protein L12 (Brot et al., Arch. Biochem. Biophys. 155:475 (1973)), in rat liver (Pestana, A. and Pitot, H. C. Biochemistry 14:1404 (1975); Green et al., Can. J. Biochem. 56:1075 (1978)); and Pestana, A. and Pitot, H. C. Biochemistry 14:1397 (1975)), calf lens (Granger et al., Proc. Nat'l Acad. Sci. U.S.A. 73:3031 (1976)), rat pituitary (Woodford, T. A. and Dixon J. E., J. Biol. Chem. 254:4993 (1979); Pease, K. A. and Dixon J. E., Arch. Biochem. Biophys. 212:177 (1981); and Glembotski, C. C., J. Biol Chem. 257:10501 (1982)), ox pituitary (Massey D. E. and Smyth, D. G., Biochem. Soc'y Trans. 8:751-753 (1980), hen oviduct (Tsunasawa et al., J. Biochem. 87:645 (1980)), and in wheat germ (Kido et al., Arch. Biochem. Biophys. 208:95 (1981)). However, isolation of this enzyme was not achieved, and only the enzyme from hen oviduct has been partially purified about 40-fold (Tsunasawa et al., J. Biochem. 87:645 (1980). The inability to isolate and purify these enzymes is due to their low concentration and extreme instability after purification.